Method and granulation system for producing fertilizer granules

Abstract

The invention relates to a method for producing fertilizer granules (2) and to a granulation system (1) suitable for carrying out the method.

Description

[0001] Process and granulation plant for the production of fertilizer granules

[0002] The invention relates to a process for producing fertilizer granules, wherein in a first step, mineral acid and a secondary raw material are mixed in a mixing chamber to form a heavy metal-rich suspension containing dissolved heavy metals, wherein in a second step, during a residence time in a reaction chamber, dissolved heavy metals in the heavy metal-rich suspension are precipitated by adding a sulfidic precipitant, wherein the precipitated heavy metals are separated in a third step by a separation device so that a phosphate-rich suspension remains, and wherein in a fourth step the fertilizer granules are produced from the phosphate-rich suspension.

[0003] Furthermore, the invention relates to a granulation plant for carrying out the method according to one of claims 1 to 12, wherein the granulation plant has a mixing chamber in which a mineral acid and a secondary raw material can be mixed to form a heavy metal-rich suspension containing dissolved heavy metals, a reaction chamber in which the heavy metals dissolved in the heavy metal-rich suspension are precipitated by adding a sulfidic precipitant, a separation device in which the precipitated heavy metals are separated so that a phosphate-rich suspension remains, and a granulation device in which fertilizer granules are produced from the phosphate-rich suspension.

[0004] Processes for the production of fertilizer granules and in particular the separation of heavy metals in such processes are already known:

[0005] DE 10 2016 116 633 A1 discloses a process for producing fertilizer granules, wherein a suspension is produced from a phosphate-containing secondary raw material and a mineral acid, and wherein the sparingly soluble phosphates of the phosphate-containing secondary raw material are at least partially dissolved in the produced suspension and / or converted into a water- and / or neutral ammonium citrate-soluble phosphate phase, and this suspension is then fed to a granulation stage. DE 10 2016 116 633 A1 further explains that the heavy metals can be separated in the process by measures known to those skilled in the art. Examples of such methods include precipitation and ion exchange.

[0006] According to the applicant's internal knowledge, the use of sodium sulfide (Na2S) in a sulfide precipitation is particularly suitable due to its good availability and low costs as well as the rapid reaction rates and the formation of a coarse flocculent precipitate and a subsequent separation of the coarse flocculent precipitate by a separation device.

[0007] Typically, the use of sulfide precipitants in acidic environments results in an increased evolution of hydrogen sulfide gas as a parallel reaction, which reduces the efficiency of heavy metal removal. Due to the toxicity of hydrogen sulfide gas and the specific requirements for plant and process safety resulting from its formation, this should be avoided.

[0008] The object of the invention is to take measures to minimize or prevent the formation of hydrogen sulfide gas in a granulation plant for carrying out a process for producing fertilizer granules and in carrying out the process itself in which heavy metals are precipitated by adding a sulfide precipitant.

[0009] The object is achieved in a method of the type mentioned at the outset in that the reaction chamber is a component of a redox potential measuring unit which has a precipitant inlet communicating with the reaction chamber, an oxidant inlet communicating with the reaction chamber, a redox potential measuring device and a control device having a control functionality, wherein a redox potential of the heavy metal-rich suspension is measured via the redox potential measuring device and transmitted to the control device, wherein a lower threshold value of the redox potential and an upper threshold value of the redox potential are stored in the control device,and wherein, to regulate the heavy metal precipitation, the control device supplies an oxidizing agent to the reaction chamber via the oxidizing agent inlet if the measured redox potential falls below the lower threshold value, and the precipitant is supplied to the reaction chamber via the precipitating agent inlet if the measured redox potential exceeds the upper threshold value, until a predefined heavy metal concentration in the heavy metal-rich suspension is undershot. Secondary raw materials preferably include phosphate-containing waste materials, such as, in particular, sewage sludge ash, preferably from the mono- or co-incineration of sewage sludge, slag, animal excrement, animal meal, animal remains and carcasses, ash from the incineration of liquid manure and digestate, wood ash, or combustion residues from biomass, each as individual substances or as mixtures thereof.

[0010] A precipitant is a substance or mixture of substances that causes a chemical reaction (substance transformation) in which a dissolved substance reacts to form an insoluble solid (the precipitate). Polysulfidic precipitants are also considered sulfidic precipitants. In particular, sodium sulfide (Na2S) is used as a sulfidic precipitant.

[0011] An oxidizing agent is a substance that oxidizes other substances while itself being reduced in the process. Examples of oxidizing agents include hydrogen peroxide (H2O2), anions of halogenated oxygen acids such as bromate (BrOs-) and hypochlorite (ClO-), especially sodium hypochlorite (NaOCl), sodium percarbonate, chlorine dioxide solution, or elements such as oxygen, sulfur, and the halogens such as fluorine, chlorine, bromine, and iodine.

[0012] In particular, the process performed in the second step is user-independent, exhibits high reproducibility, is a highly selective method since only sulfide is detected, and offers a high degree of flexibility. Furthermore, the process performed in the second step is independent of the mineral acid and the secondary raw material used in the first step.

[0013] Advantageously, the process for producing fertilizer granules carried out enables a suitably finely dosed control of precipitation processes for the reduction of heavy metal loads by means of sulfide precipitants in the reaction chamber during the second step, while simultaneously taking into account the problem of hydrogen sulfide gas evolution in acidic environments.

[0014] The residence time of the heavy-metal-rich suspension in the reaction chamber is preferably between 1 and 120 minutes. During the residence time of the heavy-metal-rich suspension in the reaction chamber, extraction of the heavy metals and phosphates from the secondary raw material and sulfide precipitation of the heavy metals advantageously take place in parallel.

[0015] Continuous dosing of the sulfide precipitant via the precipitant inlet into the reaction chamber is made possible by measuring the redox potential using the redox potential measuring device. The measured redox potential, the lower threshold value of the redox potential, and the upper threshold value of the redox potential are preferably assigned to a sulfide concentration in the control device. Particularly preferably, the formation of hydrogen sulfide gas is reduced or prevented by the appropriately finely dosed control, and at the same time, the sulfide concentration in the phosphate-rich suspension is adjusted in the second step below the limit value for wastewater from industrial and commercial operations of less than 2 mg / l.

[0016] The process opens up the production of heavy metal-depleted fertilizer granules, which preferably comply with the applicable fertilizer regulations.

[0017] Advantageous developments of the process are disclosed in the dependent claims. The second step of the process is advantageously carried out batchwise.

[0018] In addition, the precipitant inlet is arranged in the reaction chamber in such a way that the precipitant is added below a liquid surface of the heavy metal-rich suspension.

[0019] The redox potential measuring unit preferably has a gas scrubber communicating with the reaction chamber, in which hydrogen sulfide gas produced in the reaction chamber is chemically bound, expediently by a chemical reaction with a particularly strongly alkaline solution, preferably with sodium hydroxide. The gas scrubber expediently has a gas scrubber monitoring device designed as a pH measuring device, which monitors the gas scrubber's absorption capacity for hydrogen sulfide gas. This can prevent saturation of the alkaline solution, for example by adding fresh alkaline solution and discharging used alkaline solution.

[0020] According to an advantageous development, the redox potential measuring device has an ion-selective measuring electrode made of silver, on whose surface a silver sulfide layer forms during the residence time in the presence of sulfide. This allows the concentration of free, unreacting sulfide in the heavy metal-rich suspension to be measured using the redox potential in order to regulate the addition of the sulfide precipitant. The reaction of the sulfide with the silver ions of the silver electrode causes a lowering of the redox potential.

[0021] The redox potential measuring device has an ion-selective measuring electrode and a redox measuring electrode, wherein the ion-selective measuring electrode is expediently designed as a silver measuring electrode and the redox measuring electrode as a silver / silver chloride redox measuring electrode. The redox potential measuring device is preferably arranged in the reaction chamber or in a bypass device, whereby the course of the precipitation reaction can be measured. Expediently, the redox potential decreases at high concentrations of the free, unreacting sulfide in the heavy metal-rich suspension, so that the addition of the sulfidic precipitant is reduced or stopped and / or the addition of the oxidizing agent takes place. The dosage of sulfidic precipitant can thus be controlled in a targeted manner without generating an overdose of the sulfidic precipitant.

[0022] In the case of a particularly strongly negative redox potential, strong hydrogen sulfide gas formation can be assumed due to the high concentration of free, non-reacting sulfide in the heavy metal-rich suspension.

[0023] By adding the oxidizing agent, it is possible to oxidize the sulfide to the sulfate ion. This does not impair the product quality of the fertilizer granules. An overdose of oxidizing agent leads to a sharp increase in the redox potential, as the heavy metal sulfides formed are decomposed by oxidation. Accordingly, the control of the addition of sulfide precipitant and oxidizing agent in the second step is of great importance in order to produce a suitable phosphate-rich suspension for the fertilizer granules.

[0024] The process, in particular the second step, can therefore be used to specifically control both the dosage of oxidizing agent to destroy excess sulfide and the precipitation of the heavy metals by the sulfidic precipitant in order to achieve or fall below a predefined heavy metal concentration in the heavy metal-rich suspension.

[0025] The silver / silver chloride redox measuring electrode, used in parallel as a reference electrode, serves to suppress the changing redox potential of the heavy-metal-rich suspension and thus serves as a reference system for the ion-selective measuring electrode. The resulting measurement signal can thus be assigned to a sulfide concentration in the suspension. Furthermore, the temperature of the heavy-metal-rich suspension is continuously logged, so that the temperature-dependent redox potential can always be correctly evaluated.

[0026] To measure the heavy metal concentration, a sample of the heavy metal-rich suspension is taken from the reaction chamber and then analyzed. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) is preferably used to analyze the sample from the reaction chamber.

[0027] In ICP-OES analysis, the sample solution is introduced into an inductively coupled argon plasma via a pneumatic nebulizer system. At a temperature of 5000 to 7000 K in the plasma, the elements contained in the solution are atomized and excited to emit light. During simultaneous analysis, the emitted light is split into element-specific wavelengths using a polychromator, and the light intensity is detected using a series of permanently installed photomultipliers. It is also possible to record individual wavelengths in the emission spectrum using a sequentially operating high-resolution monochromator. The light intensity is proportional to the number of atoms of an element excited in the plasma. To quantitatively determine the element content of a solution, the instrument is calibrated using synthetic solutions of known content. Commercially available reference solutions are used to check the results.

[0028] Furthermore, the object is achieved in a granulation plant of the type mentioned at the outset in that the reaction chamber is a component of a redox potential measuring unit which has a precipitant inlet communicating with the reaction chamber, an oxidizing agent inlet communicating with the reaction chamber, a redox potential measuring device and a control device having a control functionality.

[0029] Secondary raw materials are, in particular, waste materials containing phosphate, such as sewage sludge ash from the mono- or co-incineration of sewage sludge, slag, animal excrement, meat and bone meal, animal remains and carcasses, ash from the incineration of liquid manure and digestate, wood ash or combustion residues from biomass as individual materials or mixtures thereof.

[0030] Moreover, sulphide precipitants are also referred to as polysulphide precipitants hereinafter.

[0031] Advantageously, the preferred process for producing fertilizer granules can be carried out on the granulation plant, with an expediently finely dosed control of precipitation processes to reduce heavy metal loads by means of sulfidic precipitants in the reaction chamber during the second step, while simultaneously taking into account the problem of hydrogen sulfide gas evolution in an acidic environment. The residence time of the heavy metal-rich suspension in the reaction chamber is preferably between 1 minute and 120 minutes. During the residence time of the heavy metal-rich suspension in the reaction chamber, an extraction of the heavy metals and phosphates from the secondary raw material and the sulfidic precipitation of the heavy metals advantageously take place in parallel.

[0032] Continuous dosing of the sulfide precipitant is made possible by measuring the redox potential using the redox potential measuring device. The measured redox potential, the lower threshold value of the redox potential, and the upper threshold value of the redox potential are preferably assigned to a sulfide concentration in the control device. Particularly preferably, the formation of hydrogen sulfide gas is reduced or prevented by the appropriately finely dosed control, and at the same time, the sulfide concentration in the phosphate-rich suspension is adjusted in the second step below the limit value for wastewater from industrial and commercial operations of less than 2 mg / l.

[0033] The process that can be carried out on the granulation plant opens up the production of heavy metal - depleted fertilizer granules that comply in particular with the applicable fertilizer regulations .

[0034] Advantageous further developments of the granulation plant emerge from the following subclaims.

[0035] Particularly preferably, the reaction chamber of the redox potential measuring unit is designed as a reaction vessel, with the mixing chamber and the reaction chamber expediently being designed as a mixing and reaction chamber. Further advantageously, the mixing chamber has a mixing device with a motor-driven stirrer, with the stirrer expediently being designed as a blade, disc, or anchor stirrer. This ideally mixes the mineral acid and the secondary raw material with one another, so that the phosphorus bound in the secondary raw material and the heavy metals in the suspension are dissolved by the mineral acid.

[0036] In particular, the redox potential measuring unit has a bypass device through which the heavy metal-rich suspension is circulated, wherein the redox potential measuring device is expediently arranged in the bypass device.

[0037] The residence time of the suspension can be adjusted in a continuously running process by means of the bypass device which circulates the heavy metal-rich suspension.

[0038] According to an additional advantageous development of the granulation plant, the redox potential measuring device has an ion-selective measuring electrode and a redox measuring electrode, wherein the ion-selective measuring electrode is expediently designed as a silver measuring electrode and the redox measuring electrode as a silver / silver chloride redox measuring electrode. During the residence time in the presence of sulfide, a silver sulfide layer forms on the surface of the ion-selective measuring electrode made of silver, whereby the concentration of the free, unreacting sulfide in the heavy metal-rich suspension can be measured using the redox potential in order to regulate the addition of the sulfidic precipitant. The reaction of the sulfide with the silver ions of the silver electrode causes a lowering of the redox potential.The silver / silver chloride redox measuring electrode, used in parallel as a reference electrode, serves to suppress the changing redox potential of the heavy-metal-rich suspension and thus serves as a reference system for the ion-selective measuring electrode. The resulting measurement signal can thus be assigned to a sulfide concentration in the suspension. Furthermore, the temperature of the heavy-metal-rich suspension is continuously logged, so that the temperature-dependent redox potential can always be accurately evaluated.

[0039] The redox potential measuring device is preferably arranged in the reaction chamber or in a bypass device, which allows the precipitation reaction to be measured. The redox potential advantageously decreases at high concentrations of free, unreacting sulfide in the heavy-metal-rich suspension, so that the addition of the sulfide precipitant is reduced or stopped and / or the addition of the oxidizing agent is carried out. The dosage of sulfide precipitant can thus be controlled in a targeted manner without generating an overdose of the sulfide precipitant.

[0040] Furthermore, the redox potential measuring unit has a gas scrubber, which expediently has a gas scrubber monitoring device. The redox potential measuring unit preferably has a gas scrubber which communicates with the reaction chamber and in which hydrogen sulfide gas produced in the reaction chamber is chemically bound, expediently by a chemical reaction with a particularly strongly alkaline solution, preferably with sodium hydroxide. In a preferred development, the gas scrubber monitoring device has a pH value measuring device for monitoring the absorption capacity of the sodium hydroxide for hydrogen sulfide gas. The gas scrubber expediently has a gas scrubber monitoring device designed as a pH value measuring device, which monitors the absorption capacity of the gas scrubber for hydrogen sulfide gas. This can be used to prevent saturation of the alkaline solution, for example by...fresh alkaline solution is fed in and used alkaline solution is discharged. In addition, the gas scrubber is preferably designed as a countercurrent device, which is expediently flowed through by hydrogen sulphide gas and sodium hydroxide using the countercurrent principle. With the countercurrent principle, two material flows - here the strongly alkaline solution and the hydrogen sulphide gas - are allowed to flow past each other from opposite directions and brought into contact with each other in such a way that the exchange of material or heat is possible between them. Since the flow of current always creates a temperature or concentration gradient between the material flows, it is possible to transfer almost the entire heat or material load from one material flow to the other.

[0041] Additionally preferably, the precipitant inlet and the oxidant inlet are formed as one inlet.

[0042] According to an advantageous development, the control device has a database in which the lower threshold value of the redox potential and the upper threshold value of the redox potential are stored. Furthermore, a predefined heavy metal concentration in the heavy-metal-rich suspension is expediently stored in the database.

[0043] The control device preferably further comprises a comparator device that compares the measured redox potential of the heavy-metal-rich suspension with the two stored threshold values. The control device advantageously comprises an evaluation device that evaluates a comparison performed in the comparator device.

[0044] By means of a control device designed in this way, an automated control of the second step can be achieved so that the formation of hydrogen sulphide can be minimized or prevented.

[0045] For granulating the fertilizer granules in the fourth step, the granulation device has a granulator which is expediently designed as a fluidized bed or spouted bed apparatus.

[0046] In a granulator designed in this way, granulation can take place as spray agglomeration and / or spray granulation. Spray granulation combines convective drying or solidification with granulate formation in a single process step. Here, compact granules are formed by spraying the suspension low in heavy metals (liquid containing solids) into a fluidized bed or spouted bed using a nozzle, in particular a pressure nozzle, which is energetically more advantageous than conventional nozzles, whereby the liquid phase evaporates and particle growth takes place due to the remaining solid. This process is often also referred to as granulation drying or build-up granulation.

[0047] Pressure nozzles are also often referred to as single-component nozzles and use their own energy to atomize the supplied liquid. Additional compressed air or steam is generally not required. The pressurized liquid exits into the surrounding gas atmosphere as a jet or lamella. Depending on the nozzle type and process parameters, a wide variety of sprays and droplet sizes are produced. In spray granulation processes, the phosphate-rich suspension is sprayed evenly onto smaller particles through spray nozzles. These smaller particles are called granulation cores and are generally made of the same material as the solid sprayed onto the liquid phase.They are formed either directly in the process itself through internal nucleation, such as spray drying or abrasion, or through the recycling of fine particles or the comminution of oversized granules from the screening-grinding cycle and therefore do not have to be added as a solid raw material.

[0048] Further advantageously, the redox potential measuring unit has a sampling device for taking a sample from the heavy metal-rich suspension to determine the heavy metal concentration. The granulation system, in particular the redox potential measuring unit, expediently has an analysis device with which the heavy metal concentration in the heavy metal-rich suspension is determined from the sample taken with the sampling device. The analysis device is expediently designed as an emission spectrometer for performing inductively coupled plasma optical emission spectroscopy (ICP-OES).

[0049] In ICP-OES analysis, the sample solution is introduced into an inductively coupled argon plasma via a pneumatic nebulizer system. At a temperature of 5000 to 7000 K in the plasma, the elements contained in the solution are atomized and excited to emit light. During simultaneous analysis, the emitted light is split into element-specific wavelengths using a polychromator, and the light intensity is detected using a series of permanently installed photomultipliers. It is also possible to record individual wavelengths in the emission spectrum using a sequentially operating high-resolution monochromator. The light intensity is proportional to the number of atoms of an element excited in the plasma. To quantitatively determine the element content of a solution, the instrument is calibrated using synthetic solutions of known content. Commercially available reference solutions are used to check the results.

[0050] Finally, the separation device is preferably designed as a filter, a centrifuge, or a suction device. This allows the precipitated heavy metals to be separated from the heavy-metal-rich suspension and remain as a residue alongside the phosphate-rich suspension.

[0051] The invention is explained in more detail below using the attached drawing, which shows

[0052] Figure 1 is a process flow diagram of a first embodiment of the process for producing fertilizer granules,

[0053] Figure 2 is a process flow diagram of a second embodiment of the process for producing fertilizer granules and

[0054] Figure 3 is a diagram of a measured redox potential over time according to a process for producing fertilizer granules according to Fig. 1 carried out on a granulation plant.

[0055] Unless otherwise stated, the following description refers to all embodiments of a granulation plant 1 illustrated in the drawing for carrying out a process for producing fertilizer granules 2 .

[0056] Fig. 1 shows a process flow diagram of a first embodiment of the process for producing fertilizer granules 2, which is carried out on a granulation plant 1.

[0057] The granulation plant 1 shown in Fig. 1 has a mixing chamber 4 designed as a mixing container 3, wherein a mineral acid 5 can be fed to the mixing chamber 4 via a mineral acid inlet 6 and a secondary raw material 7 can be fed via a secondary raw material inlet 8.

[0058] Mineral acids 5 are the four strong inorganic acids hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid or a mixture of these.

[0059] Secondary raw materials are phosphate-containing waste materials, such as sewage sludge ash from the mono- or co-incineration of sewage sludge, slag, animal excrement, meat and bone meal, animal remains and carcasses, ash from the incineration of liquid manure and digestate, wood ash or combustion residues from biomass, each as an individual material or as mixtures thereof.

[0060] In a first step of the process, the mineral acid 5 and the secondary raw material 7 are mixed together in the mixing chamber 4 to form a suspension 9, so that the phosphorus bound in the secondary raw material 7 and the heavy metals dissolve in a suspension 9 by the mineral acid 5. The suspension 9 is also referred to below as a heavy metal-rich suspension 9 containing dissolved heavy metals. For this purpose, a mixing device 11 having a motor-driven stirrer 10 is arranged in the mixing chamber 4. The stirrer 10 is designed as a blade stirrer 12 in the first embodiment.

[0061] From the mixing chamber 4, the heavy metal-rich suspension 9 is transferred to the reaction chamber 13. The residence time of the heavy metal-rich suspension 9 in the reaction chamber 13 is between 1 minute and 120 minutes. The reaction chamber 13 is a component of the redox potential measuring unit 15 designed as a reaction vessel 14.

[0062] The redox potential measuring unit 15 has a precipitant inlet 16 communicating with the reaction chamber 13, an oxidizing agent inlet 17 communicating with the reaction chamber 13, a redox potential measuring device 18, and a control device 19 having a closed-loop control functionality. Furthermore, the redox potential measuring unit 15 has a bypass device 20 through which the heavy metal-rich suspension 9 is circulated, with the redox potential measuring device 18 being arranged in the bypass device 20. The residence time of the heavy metal-rich suspension 9 in the reaction chamber 13 can be adjusted as desired by the bypass device 20 associated with the redox potential measuring unit 15.

[0063] The redox potential measuring device 18 has an ion-selective measuring electrode 21 and a redox measuring electrode 22. In this embodiment, the ion-selective measuring electrode 21 is designed as a silver measuring electrode 23 and the redox measuring electrode 22 is designed as a silver / silver chloride redox measuring electrode 24. Other pairings of ion-selective measuring electrode 21 and redox measuring electrode 22 are used in other embodiments not shown for measuring the redox potential.

[0064] The redox potential measuring device 18 measures the redox potential of the suspension 9 which is low in heavy metals and then transmits this value to the control device 19. The measured redox potential, the lower threshold value of the redox potential and the upper threshold value of the redox potential are each assigned to a sulfide concentration in the control device 19. The redox potential (also known as redox voltage or oxidation-reduction potential, ORP for short) is the measurement of the capacity of an aqueous system to absorb or release electrons from a chemical reaction. Oxidation causes the release of electrons, reduction causes the absorption of electrons. The redox potential of a system can therefore be changed by adding new species if these have a reductive or oxidative effect.

[0065] The control device 19 has a database 25 in which the lower threshold value of the redox potential and the upper threshold value of the redox potential are stored. The control device 19 also has an evaluation device 27 which evaluates a comparison carried out in the comparison device 26, namely as to where the measured redox potential lies in the respective comparison with the lower or upper threshold value. In addition, the control device 19 further has a comparison device 26 which compares the measured redox potential of the heavy metal-rich suspension 9 with the two stored threshold values.

[0066] During the residence time in the presence of sulfide, a silver sulfide layer forms on the surface of the ion-selective measuring electrode 21 made of silver, whereby the concentration of the free, non-reacting sulfide in the heavy metal-rich suspension 9 can be measured by the redox potential in order to regulate the addition of the sulfide precipitant 29.

[0067] To regulate the heavy metal precipitation in the heavy metal-rich suspension 9 contained in the reaction chamber 13, an oxidizing agent 28 is fed to the reaction chamber 13 via the oxidizing agent inlet 17 by the control device 19 when the measured redox potential falls below the lower threshold value, and the precipitant 29 is fed to the reaction chamber 13 via the precipitant inlet 16 when the measured redox potential exceeds the upper threshold value, until a predefined heavy metal concentration in the heavy metal-rich suspension 9, which is further stored in the database 25, is undershot. The redox potential decreases accordingly at high concentrations of the free, unreacting sulfide in the heavy metal-rich suspension 9, so that the addition of the sulfidic precipitating agent 29 is reduced or stopped and / or the addition of the oxidizing agent 28 takes place.The dosing of the oxidizing agent 28 and the precipitating agent 29 takes place below a liquid surface 61 of the heavy metal-rich suspension 9 , directly into it.

[0068] In addition to falling below the predefined heavy metal concentration in the heavy metal-rich suspension 9, the redox potential associated with the sulfide concentration in the heavy metal-poor suspension 9 is adjusted in a second step so that the sulfide concentration is less than 2 mg / l. This ensures that the sulfide concentration is below a limit concentration of 2 mg / l for wastewater from industry and commerce.

[0069] The redox potential measuring unit 15 also has a sampling device 30 for taking a sample 31 from the heavy metal-rich suspension 9 contained in the reaction chamber 13 and for further determining a heavy metal concentration in the sample 31 in an analysis device 32.

[0070] The sample 31 is analyzed by means of the analysis device 32. In the embodiment, the granulation system 1, in particular the redox potential measuring unit 15, has an analysis device 32 designed as a spectrometer 33. The spectrometer 33 is designed as an emission spectrometer 34 for performing optical emission spectrometry with inductively coupled plasma, with which the heavy metal concentration in the heavy metal-rich suspension 9 is determined from the sample 31 taken with the sampling device 30.

[0071] Furthermore, the redox potential measuring unit 15 has a gas scrubber 35 communicating with the reaction chamber 13, which expediently has a gas scrubber monitoring device 36. In the gas scrubber 35, hydrogen sulfide gas 37 produced in the reaction chamber 13 is chemically bound, expediently by a chemical reaction with a particularly strongly alkaline solution 38, preferably with sodium hydroxide 39. The gas scrubber 35 is designed as a countercurrent device 40, which expediently has hydrogen sulfide gas

[0072] 37 and sodium hydroxide 39 flow through it in countercurrent.

[0073] In the embodiment shown in Fig. 1, the gas scrubber monitoring device 36 has a pH measuring device 41 for monitoring the absorption capacity of the sodium hydroxide

[0074] 38 for hydrogen sulfide gas 37. This allows the alkaline solution 38 to be run in the circuit 62, wherein used solution 60 can be removed from the circuit 62 and fresh alkaline solution 38 in the form of sodium hydroxide 39 can be fed in. The absorption capacity can be monitored at any time by the pH measuring device 41, in particular a pH probe 42. After the hydrogen sulfide gas 37 has been purified by the acid-base reaction, sodium hydrogen sulfide in aqueous used solution 60 emerges from the gas scrubber 35. In the second step, during the residence time in the reaction chamber 13, heavy metals dissolved in the heavy metal-rich suspension 9 are precipitated by adding the sulfidic precipitant 29.In a third step, the precipitated heavy metals are separated from the heavy metal-rich suspension 9 by a separation device 43, so that, in addition to a heavy metal residue 44, a phosphate-rich suspension 45 remains. In the first embodiment, the separation device 43 is designed as a filter 46.

[0075] While the heavy metal residue 44 is subjected to further processing (not shown) after the third step, the fertilizer granules 2 are produced from the phosphate-rich suspension 45 in a fourth step. The fertilizer granules 2 are produced in the granulation plant 1 in a granulation device 59 designed as a granulator 47, wherein the granulator 47 is designed as a fluidized bed apparatus 48 in the first embodiment.

[0076] Fig. 2 shows a process flow diagram of a second embodiment of the process for producing fertilizer granules 2, which is carried out on a further granulation plant 1.

[0077] In the second embodiment, in contrast to the first embodiment shown in Fig. 1, the first step and the second step take place in a space 49. Accordingly, the mixing space 4 and the reaction space 13 are designed as a mixing and reaction space 49, with the mixing and reaction space 49 simultaneously being designed as a mixing and reaction container 50.

[0078] A mineral acid 5 is fed to the mixing and reaction chamber 49 via a mineral acid inlet 6, a secondary raw material 7 via a secondary raw material inlet 8 and, expediently, additives 51, such as water and / or organic raw materials and / or ammonia, struvite or the like, via an additive inlet 52.

[0079] Mineral acids 5 are the four strong inorganic acids hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid or any mixture thereof.

[0080] Secondary raw materials are phosphate-containing waste materials, such as sewage sludge ash from the mono- or co-incineration of sewage sludge, slag, animal excrement, meat and bone meal, animal remains and carcasses, ash from the incineration of liquid manure and digestate, wood ash or combustion residues from biomass, each as an individual material or as mixtures thereof.

[0081] In a first step of the process, the mineral acid 5 and the secondary raw material 7 and, expediently, the additives 51 are mixed batchwise with one another to form a suspension 9 in the mixing and reaction chamber 49, such that the phosphorus bound in the secondary raw material 6 and the heavy metals in the suspension 9 are dissolved by the mineral acid 5. The suspension 9 is also referred to below as a heavy metal-rich suspension 9 containing dissolved heavy metals. For this purpose, a mixing device 11 having a motor-driven stirrer 10 is arranged in the mixing and reaction chamber 49. The stirrer 10 is designed as an anchor stirrer 53 in the second embodiment.

[0082] Parallel to the mixing process in the first step, the second step, namely heavy metal precipitation, also takes place in the mixing and reaction chamber 49. The residence time of the heavy metal-rich suspension 9 in the reaction chamber 13 is between 1 minute and 120 minutes. The mixing and reaction chamber 49 is a component of the redox potential measuring unit 15, which is designed as a mixing and reaction vessel 50.

[0083] The redox potential measuring unit 15 has a precipitant inlet 16 communicating with the mixing and reaction chamber 49, an oxidant inlet 17 communicating with the mixing and reaction chamber 49, a redox potential measuring device 18, and a control device 19 having a control functionality. Furthermore, the redox potential measuring unit 15 has a sampling device 30, an analysis device 32, and a gas scrubber 35.

[0084] The operation and structure of the control device 19, the sampling device 30, the analysis device 32 and the gas scrubber 35 of the redox potential measuring device 15 of the second embodiment correspond to the control device 19, the sampling device 30, the analysis device 32 and the gas scrubber 35 of the redox potential measuring device 15 shown in the first embodiment.

[0085] In contrast to the first embodiment, the redox potential measuring unit 15 does not have a bypass device 20.

[0086] In the first and second steps, which run in parallel, heavy metals dissolved in the heavy-metal-rich suspension 9 are precipitated by adding the sulfide precipitant 29 during the residence time in the mixing and reaction chamber 49. In a third step, the precipitated heavy metals are separated from the heavy-metal-rich suspension 9 by a separation device 43, so that, in addition to a heavy metal residue 44, a phosphate-rich suspension 45 remains. In the first embodiment, the separation device 43 is designed as a suction device 54.

[0087] While the heavy metal residue 44 is fed to a further processing step (not shown) after the third step, the fertilizer granules 2 are produced from the phosphate-rich suspension 45 in a fourth step. Before the fertilizer granules 2 are granulated, additives can be added to the phosphate-rich suspension 45 via an additive inlet (not shown), for example to produce multi-component fertilizers. Other macronutrients (potassium / calcium / magnesium salts, nitrogen-containing components and / or sulfur in the form of sulfates / sulfites or elemental sulfur) are preferably added to the phosphate-rich suspension 45 as additives before granulation. The fertilizer granules 2 are produced in the granulation plant 1 in a granulation device 59 designed as a granulator 47, wherein the granulator 47 is designed as a spouted bed apparatus 55 in the first embodiment.

[0088] Fig. 3 shows a diagram of a measured redox potential E over the residence time t according to a process carried out on a granulation plant 1 for producing fertilizer granules 2 according to Fig. 1.

[0089] The measured redox potential E corresponds to a sulfide concentration in the heavy metal-rich suspension 9, wherein at time t 0 sulfidic precipitant 29 in the form of sodium sulfide (a2S) has been added. From time t 1 at which the redox potential E becomes negative, the heavy metal-rich suspension 9 contains a high concentration of free sulfide. Due to the addition of sodium sulfide, the redox potential E decreases until it becomes negative at time t 1 and subsequently even falls below the lower threshold value 56 at time t2. The addition of the sulfidic precipitant 29 is stopped and oxidizing agent 28 is added.

[0090] By adding the oxidizing agent 28, here in the form of a hydrogen peroxide solution (H202 solution), at time t2, the redox potential E of the heavy metal-rich suspension 9 increases again, which indicates the decomposition of the free sulfide.

[0091] At time ts, the redox potential E reaches the upper threshold value 57, at which the supply of oxidizing agent 28 is stopped and sulfide precipitant 29 is again added. By exceeding the upper threshold value 57, the concentration falls below the limit of 2 mg / l for wastewater from industry and commerce.

[0092] At time t4, the predefined heavy metal concentration 58 is reached, so that the control is terminated and the heavy metal residue 44 is separated from the heavy metal-rich suspension 9 in the third step in the separation device 43, so that a phosphate-rich suspension 45 remains, which is then subsequently granulated in the granulation device 59.

Claims

Claims 1. A process for producing fertilizer granules (2), wherein, in a first step, mineral acid (5) and a secondary raw material (7) are mixed in a mixing chamber (4) to form a heavy metal-rich suspension (9) containing dissolved heavy metals, wherein, in a second step, during a residence time (t) in a reaction chamber (14), dissolved heavy metals in the heavy metal-rich suspension (9) are precipitated by adding a sulfidic precipitant (29), wherein, in a third step, the precipitated heavy metals are separated by a separating device (43), so that a phosphate-rich suspension (45) remains, and wherein, in a fourth step, the fertilizer granules (2) are produced from the phosphate-rich suspension (45), characterized in that the reaction chamber (14) is a component of a redox potential measuring unit (15) which, via a precipitant inlet (16) communicating with the reaction chamber (14),an oxidizing agent inlet (17) communicating with the reaction chamber (14), a redox potential measuring device (18) and a control device (19) having a control functionality, wherein a redox potential (E) of the heavy metal-rich suspension (9) is measured via the redox potential measuring device (18) and transmitted to the control device (19), wherein a lower threshold value (56) of the redox potential (E) and an upper threshold value (57) of the redox potential (E) are stored in the control device (19), and wherein for the control of the heavy metal precipitation by the control device (19), If the measured redox potential (E) falls below the lower threshold value (56), an oxidizing agent (28) is fed to the reaction chamber (14) via the oxidizing agent inlet (17) and if the measured redox potential (E) exceeds the upper threshold value (57), the precipitant (29) is fed to the reaction chamber (14) via the precipitant inlet (16) until a predefined heavy metal concentration (58) in the heavy metal-rich suspension (9) is undershot.

2. Process according to claim 1, characterized in that in particular the second step of the process is carried out batchwise.

3. Process according to claim 1 or 2, characterized in that the residence time (t) in the reaction chamber is between 1 min and 120 min.

4. Method according to one of the preceding claims, characterized in that the measured redox potential (E), the lower threshold value (56) of the redox potential (E) and the upper threshold value (57) of the redox potential (E) are assigned to a sulfide concentration in the control device (19).

5. Method according to one of the preceding claims, characterized in that the precipitant inlet (16) is arranged in the reaction chamber (14) in such a way that the precipitant (29) is metered in below a liquid surface (61) of the heavy metal-rich suspension (9).

6. Method according to one of the preceding claims, characterized in that the redox potential measuring unit (15) is connected to the reaction chamber (14) via a Gas scrubber (35) in which hydrogen sulphide (37) produced in the reaction chamber (14) is chemically bound, expediently by a chemical reaction with a particularly strongly alkaline solution (38), preferably with sodium hydroxide (39).

7. The method according to claim 6, characterized in that the gas scrubber (35) has a gas scrubber monitoring device (36) designed as a pH value measuring device (41), which monitors an absorption capacity for hydrogen sulfide gas (37) of the gas scrubber (35).

8. Method according to one of the preceding claims, characterized in that the sulfide concentration in the phosphate-rich suspension (45) is adjusted in the second step, wherein the sulfide concentration is expediently less than 2 mg / l.

9. Method according to one of the preceding claims, characterized in that the redox potential measuring device (18) has an ion-selective measuring electrode (21) made of silver, on the surface of which a silver sulfide layer forms during the residence time (t) in the presence of sulfide, whereby a concentration of the free, non-reacting sulfide in the heavy metal-rich suspension (9) can be measured by the redox potential (E) in order to regulate the addition of the sulfidic precipitant (29).

10. The method according to claim 9, characterized in that the redox potential (E) decreases at high concentrations of the free, non-reacting sulfide in the heavy metal-rich suspension (9), so that the addition of the sulfidic precipitant (29) is reduced or stopped and / or the addition of the oxidizing agent (28) takes place.

11. Method according to one of the preceding claims, characterized in that in order to measure the heavy metal concentration (58) a sample (31) of the heavy metal-rich suspension (9) is taken from the reaction chamber (14) and subsequently analyzed.

12. The method according to claim 11, characterized in that an optical emission spectrometry with inductively coupled plasma is used to analyze the sample (31) from the reaction chamber (14).

13. Granulation plant (1) for carrying out the method according to one of claims 1 to 12, wherein the granulation plant (1) comprises a mixing chamber (4) in which a mineral acid (5) and a secondary raw material (7) can be mixed to form a heavy metal-rich suspension (9) containing dissolved heavy metals, a reaction chamber (14) in which the heavy metals dissolved in the heavy metal-rich suspension (9) are precipitated by adding a sulfidic precipitant (29), a separation device (43) in which the precipitated heavy metals are separated so that a phosphate-rich suspension (45) remains, and a granulation device (59) in which fertilizer granules (2) are produced from the phosphate-rich suspension (45), characterized in that the reaction chamber (14) is a component of a redox potential measuring unit (15) which is connected to the reaction chamber (14) via a Precipitant inlet (16) ,an oxidizing agent inlet (17) communicating with the reaction chamber (14), a redox potential measuring device (18) and a control device (19) having a control functionality.

14. Granulation plant (1) according to claim 13, characterized in that the reaction space (14) of the redox potential measuring unit (15) is designed as a reaction container (13).

15. Granulation plant (1) according to claim 13 or 14, characterized in that the mixing chamber (4) and the reaction chamber (14) are designed as a mixing and reaction chamber (49).

16. Granulation plant (1) according to one of claims 13 to 15, characterized in that the mixing chamber (4) has a mixing device (10) having a motor-driven stirrer (9), wherein the stirrer (9) is expediently designed as a blade (11) or disc or anchor stirrer (53).

17. Granulation plant (1) according to one of claims 13 to 16, characterized in that the redox potential measuring unit (15) has a bypass device (20) via which the heavy metal-rich suspension (9) is circulated, wherein the redox potential measuring device (18) is expediently arranged in the bypass device (20).

18. Granulation plant (1) according to one of claims 13 to 17, characterized in that the redox potential measuring device (18) has an ion-selective measuring electrode (21) and a redox measuring electrode (22), wherein the ion-selective measuring electrode (21) is expediently designed as a silver measuring electrode (23) and the redox measuring electrode (22) is designed as a silver / silver chloride redox measuring electrode (24).

19. Granulation plant (1) according to one of claims 13 to 18, characterized in that the redox potential measuring unit (15) has a gas scrubber (35) which expediently has a gas scrubber monitoring device (36).

20. Granulation plant (1) according to claim 19, characterized in that the gas scrubber (35) is designed as a countercurrent device (40) through which hydrogen sulfide gas (37) and sodium hydroxide (39) expediently flow in countercurrent.

21. Granulation plant (1) according to claim 19 or 20, characterized in that the gas scrubber monitoring device (36) has a pH value measuring device (41) for monitoring the absorption capacity of the sodium hydroxide (39) for hydrogen sulfide gas (37).

22. Granulation plant (1) according to one of claims 13 to 21, characterized in that the precipitant inlet (16) and the oxidant inlet (17) are designed as one inlet.

23. Granulation plant (1) according to one of claims 13 to 22, characterized in that the control device (19) has a database (25) in which the lower threshold value (56) of the redox potential (E) and the upper threshold value (57) of the redox potential (E) are stored.

24. Granulation plant (1) according to claim 23, characterized in that a predefined heavy metal concentration (58) in the heavy metal-rich suspension (9) is also stored in the database (25).

25. Granulation plant (1) according to claim 23 or 24, characterized in that the control device (19) further comprises a comparator device (26) which is arranged in a Comparison of the measured redox potential (E) of the heavy metal-rich suspension (9) with the two stored threshold values ​​(56,57).

26. Granulation plant (1) according to claim 25, characterized in that the control device (19) has an evaluation device (27) which evaluates a comparison carried out in the comparison device (26).

27. Granulation plant (1) according to one of claims 13 to 26, characterized in that the granulation device (59) has a granulator (47) which is expediently designed as a fluidized bed (48) or as a spouted bed apparatus (55).

28. Granulation plant (1) according to one of claims 13 to 27, characterized in that the redox potential measuring unit (15) has a sampling device (30) for taking a sample (31) from the heavy metal-rich suspension (9) for determining a heavy metal concentration (58).

29. Granulation plant (1) according to claim 28, characterized in that the granulation plant (1), in particular the redox potential measuring unit (15), has an analysis device (32) with which the determination of the heavy metal concentration (58) in the heavy metal-rich suspension (9) from the sample (31) taken with the sampling device (30) is carried out, wherein the analysis device (32) is expediently designed as an emission spectrometer (33) for carrying out an optical emission spectrometry with inductively coupled plasma.

30. Granulation plant (1) according to one of claims 13 to 29, characterized in that the separating device (43) as a filter (46) or a centrifuge or a suction device (54).

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